Generally, NOx emitted from large-scale boilers or nitric acid plants can be effectively removed through NH3—SCR which supplies ammonia as a reducing agent to a catalyst bed composed of titania-supported vanadia or iron-containing zeolite. Ammonia is highly reactive and selective and is thus very effective for removing NOx from the exhaust gas of fixed facilities even in the presence of O2. However, the use of ammonia to remove NOx from diesel exhaust gas is very dangerous because a diesel vehicle should be driven in a state of always being loaded with ammonia which is highly toxic. So, aqueous urea is used instead of ammonia as a reducing agent therein. The urea is decomposed into ammonia and carbon dioxide in the catalyst bed so that NOx is reduced to N2. Although the urea-SCR method is advantageous because NOx removal performance is high, it is problematic in that a tank for storing aqueous urea and a device for spraying such urea should be additionally mounted to a diesel vehicle. As has been done for fuel, a sales network of aqueous urea should be constructed. As well, the urea-SCR method is difficult to apply to a diesel vehicle, due to problems including low solubility of urea, freezing, and ammonia slip.
Among the SCR methods, H2—SCR using H2 as a reducing agent instead of the aqueous urea is receiving attention because the construction of an apparatus thereof is simple and there is no concern about secondary pollution. However, it is difficult to construct the supply network of H2 and to load it into a vehicle. Further, O2 in the diesel exhaust gas may first react with H2, undesirably lowering NOx selective removal efficiency by H2. Thus, the application of the above method has not been considered to date. The reason is described below.
The temperature and O2 content of diesel exhaust gas greatly vary depending on driving conditions of vehicles. During normal high-speed driving, the temperature may be 300° C. or higher and also the O2 content may exceed 10% under lean burn. Further, in order to allow NO2 to be directly reduced to N2 by H2, H2 should be strongly activated. In this case, however, a probability of reacting such H2 with O2 is increased, undesirably lowering the NOx removal efficiency. Namely, to increase the NOx removal efficiency by H2, the probability of reacting H2 with O2 should be inhibited while increasing the degree of activation of H2, which is difficult. Hence, limitations are imposed on applying the H2—SCR method to diesel vehicles.